Are your iOS-devices making changes to records that are used by desktop-clients?
If that is the case, you may want to rethink how records are being updated. If you change a lot of records that are used on the desktops as well, the server can "hang up" or "slow down a lot" and thus clients wil also slow down. Sometimes fmserver needs to be restarted, because of this.
Maybe you can run a schedule that updates records for you in stead of having the iOS-devices do the updates.
My experience with this same issue is that for 10 field-engineers the customer used a list of locations that should be shown on a (google-)map in the webviewer. In order to generate the pins on the map, the locations had to be numbered. They solved it by replacing data in just one field in as little as 25 records (in a full set of 2500) and that actually caused a lot of hangups etc. They were connected by WiFi and via Cellular/VPN
I changed this field with locations for a field with an unstored calculation, et voilá, no more hangups.